The present invention relates to an improved method and apparatus for soldering individual terminals of a surface mount connector to respective contact pads disposed on a surface of a printed circuit board. Although the description set forth below specifies solder as the joining material, it is to be understood that any fusible electrically conductive material, such as doped conductive plastic material, may be employed.
Connectors having terminals with solder tails extending therefrom for reception in plated through holes of a printed circuit board are well-known. Relatively recently, in the interests of facilitating automated contact placement and economy of circuit board manufacture, surface mount connectors have been developed with terminals having solder tails formed for disposition against respective plated contact pads on the surface of the board. The terminals are typically fixed in the connector housing, and the tails are formed to sufficient length to assure compliance with the pads regardless of any tendency of the board to warp. Typically, each solder tail is individually manually soldered to a respective contact pad by any of a variety of known techniques. The repetitive soldering operations are both time consuming and costly.
There have been a number of prior art attempts to automatically solder multiple conductors, such as solder tails of a surface mount connector, to respective contact pads on a printed circuit board. In this regard reference is made to U.S. Pat. No. 4,484,704 (Grassauer et al) and the prior art described therein. In the Grassauer patent and other prior art soldering procedures for forming multiple solder joints simultaneously, a soldering tool is employed to deliver the necessary thermal energy over a large continuous area spanning all of the connection sites. Upon energization, the soldering tool heats up until it overshoots a control temperature before settling down to that temperature. The control temperature is typically chosen somewhat above the ideal soldering temperature in order to compensate for less than ideal thermal energy transfer. This approach to thermal energy delivery has a number of disadvantages. One such disadvantage is damage to components resulting from overheating. For example, the thermal overshoot inherent in the heating tool can damage components disposed between the connection sites within the area heated by the tool. In some cases the overshoot may cause damage to the polymeric materials, insulating materials and adhesives at the connection site. It is tempting to suggest that the operator of the soldering tool might avoid the thermal overshoot by either removing the tool before the overshoot occurs or delaying application of the tool until after the overshoot occurs. This is impractical for a number of reasons. First, there is no evident indication as to when the thermal overshoot occurs. Second, although the tool warm-up time is relatively long, the time interval during which the tool temperature is sufficient to melt solder, but prior to overshoot, is too short to reliably complete the soldering operation. Further, where the tool is also employed to apply pressure to the connection site, the power must be turned off after the solder melts while pressure is maintained on the tool until the solder solidifies. The tool must be re-energized to effect the next soldering cycle. These repeated on-off cycles change the starting temperature for the transient overshoot in each cycle, thereby making it virtually impossible to determine when the tool attains the final control temperature.
Another problem associated with prior art techniques for simultaneously joining multiple solder tails of a surface mount connector to respective contact pads of a printed circuit board relates to positional alignment of the components during soldering. It is not only necessary for the multiple solder tails to be simultaneously aligned with respective multiple contact pads on the printed circuit board surface; in addition, the solder tails and contact pads must also be simultaneously aligned with multiple respective sections of the solder delivery unit. The resulting alignment procedure is often unwieldy at best.
It is desirable, therefore, to provide a method and apparatus for simultaneously joining multiple solder tails of an electrical surface mount connector to multiple respective contact pads of a printed circuit board surface without requiring a third component to be aligned at each connection site. In addition, it is desirable that the thermal energy required to melt the solder be available virtually instantaneously after energization of the heater, and that the heater be arranged to provide no more thermal energy than is required to melt the solder employed for the various connection sites. Finally, it is desirable that the apparatus have relatively low mass in order that it may cool down quickly after a soldering operation.
The present invention makes use of a relatively new automatic self-regulating heater technology disclosed in U.S. Pat. Nos. 4,256,945 (Carter et al), 4,623,401 (Derbyshire et al), 4,659,912 (Derbyshire), 4,695,713 (Krumme), 4,701,587 (Carter et al), 4,717,814 (Krumme) and 4,745,264 (Carter). The disclosures in these patents are expressly incorporated herein by reference. A heater constructed in accordance with this technology, hereinafter referred to as a self-regulating heater, employs a substrate of copper, copper alloy, or other material of low electrical resistivity, negligible magnetic permeability and high thermal conductivity. A thin surface layer of thermally-conductive magnetic material is deposited on all or part of one surface of the substrate, the surface layer material typically being an iron, nickel or nickel-iron alloy, or the like, having a much higher electrical resistance and magnetic permeability than the substrate material. The thickness of the surface layer is approximately one skin depth, based on the frequency of the energizing current and the resistance and permeability of the surface layer. A constant amplitude, high frequency alternating energizing current is passed through the heater and, as a result of the skin effect phenomenon, is initially concentrated in one skin depth corresponding to the thickness of the magnetic material. When the temperature at any point along the heater reaches the Curie temperature of the magnetic material, the magnetic permeability of the magnetic material at that point decreases dramatically, thereby significantly increasing the skin depth so that the current density profile expands into the non-magnetic substrate of low resistivity. The overall result is a lower resistance and lesser heat dissipation. If thermal sinks or loads are placed in contact with the heater at different locations along the heater length, thermal energy is transferred to the loads at those locations with the result that the temperature of the magnetic material does not rise to the Curie temperature as quickly as those locations as it does in the non-loaded locations. The constant amplitude current remains concentrated in the higher resistance surface layer at the loaded locations which dissipate considerably more resistive heating energy than is dissipated in the non-load locations where the current is distributed in the low resistance substrate.